The present invention relates to a printing wire for a dot matrix type printer head.
The following three points have been demanded for material characteristics as a printing wire used in a dot matrix type printer head:
(1) The wire has high wear resistance. Since a tip end of the wire hits an ink ribbon over 10.sup.8 times, the tip end is apt to be worn out, resulting in unclear printing. In addition, since the wire is always in sliding relation to a guide, a side wall of the wire is apt to be worn out, which leads to a poor printing precision. PA1 (2) The wire is light in weight. The printing wire must move at a high speed. Thus, in order to increase the printing speed, it is necessary for the wire to be light in weight. PA1 (3) The wire has high toughness. Since the wire is disposed to be cured by an intermediate guide, the wire is apt to be broken during the assembling of the wire or the printing operation thereof. Accordingly, high toughness is needed for the wire. PA1 i) the material according to the invention relates to a high Cr high speed tool steel containing 7.5 to 12.0% Cr. However, if the amount of C is high, there occurs a large amount of carbide readily solid-solutioned in the form of Cr.sub.23 C.sub.8 with the result that the solid-solution amount of C in the matrix increases due to the quenching heat treatment to thereby increase the amount of the residual austenite drastically. When the fine wire having a diameter of 0.2 to 0.3 mm to which the invention pertains is subjected to the quenching-tempering heat treatment, the material in which such large amount of the residual austenite is formed causes an extremely large heat treatment deformation (bending) and almost all the material is faulty. In addition, such material has a low bending strength and can not be put into practical use. PA1 ii) Since the hardness of the material is not sufficiently decreased to make the ductility be inferior even when it is subjected to annealing, the wire drawability that is important for the dot wire is degraded. As a result, it is impossible to produce the fine wires of 0.2 to 0.3 mm in diameter on an industrial scale. PA1 iii) More carbon reacts with Cr to increase carbide, and on the contrary, the amount of the solid solution of Cr into the matrix is reduced, resulting in substantial loss of the corrosion resistance. Thus, the object of the invention can not be attained.
The printing wire of this type is made, in general, of a fine wire of a cemented carbide, a tungsten fine wire, a fine wire of a high speed tool steel or the like. In these materials, the cemented carbide is superior in wear resistance but its specific weight is large so that the wire is heavy in weight. Therefore, this is not suitable for high speed printing. In addition, since its toughness is low, the wire of the cemented carbide is apt to be damaged or broken down during the assembling work, with the result that the reliability of the wire is not satisfactory.
The tungsten wire has such problems that its specific weight is large, fibrous structure is developed due to high degree plastic working so that the wire is apt to be cracked longitudinally into two halves, and that the abrasion resistance is not satisfactory.
A high speed tool steel classified in JIS SKH51 (corresponding to AISI M2) has a small specific weight which is about half the specific weight of the cemented carbide or tungsten, and has also a high toughness. In addition, in the high speed tool steel, it is possible to obtain a hardness of Hv700 to Hv900. In addition, since a suitable amount of non-solid-solutioned carbide is dispersed in the matrix, its abrasion resistance is high. Accordingly, the material has been frequently used as a wire for a dot printer wire.
The high speed tool steels are classified into two kinds in accordance with a production process, i.e., a first one produced through a conventional melting method and a second one produced through a powder metallurgy method. Since the amount of carbon and the amount of carbide forming elements can be increased in the high speed tool steel produced according to the powder metallurgy method, the wear resistance is increased. Accordingly, the high speed tool steel produced through the powder metallurgy method has been more frequently used. However, this material has problems regarding workability for making fine wires. As a result, the powder metallurgy high speed tool steel available on the market is restricted substantially to two kinds of materials, i.e., 1.3C--4Cr--6W--5Mo--3V--8Co and 2.0C--4Cr--8W--4Mo--6V--6Co. However, in order to meet the higher speed requirement and the longer service life of the printer, these powder metallurgy high speed tool steels are still not satisfactory.
Regarding prior art attempts to enhance the wear resistance of the printing wire, for example, Japanese Patent Unexamined Publication No. 52-110121 discloses a method in which a chip of cemented carbide is bonded to a tip end of the wire, Japanese Patent Unexamined Publication No. 54-54713 discloses a method in which impact quenching is effected at the tip end of the wire by using laser irradiation or the like, and Japanese Patent Unexamined Publication No. 52-96119 discloses a method in which a surface of the wire is coated with hard composite material through chemical vapour deposition process.
In the conventional methods for enhancing the abrasion resistance of the printing wire, each of the method in which the chip of cemented carbide is bonded to the tip end of the wire, the method in which the impact quenching is effected at the end portion by the laser irradiation or the like, and the method in which the wire surface is coated with hard composite material through the chemical vapour deposition process, has such disadvantages that each of the methods is not suitable for mass-production, resulting in high cost. At present, these methods have not been put into industrial use.
The present inventors observed and searched the abrasion state of the wire tip end in order to enhance the service life of the printing wire. As a result, it has been found that the wear due to the corrosion occurs at the wire tip end simultaneously together with the abrasive wear due to graphite fine particles which are pigments or dyes contained in the ink. It is considered that this is caused by the corrosion effect occurring at the wire material due to specific fatty acid ranging from several % to several tens % which fatty acid is contained in the ink. It is therefore found that it is necessary to use a wire material which is superior in corrosion resistance and well as abrasive wear resistance, in order to enhance the service life of the printing wire.
Based upon the above-described findings, the present inventors filed Japanese Patent Unexamined Publication. No. 1-83643 and Japanese Patent Application No. 63-332156 proposing the improvement of the corrosion resistance of the dot wire material by increasing he content of Cr. However, the present inventors have studied whether or not these material could meet the various requirements for the dot wire. As a result, it has been found that the material set forth in Japanese Patent Unexamined Publication No. 1-83643 shows a satisfactory corrosion resistance but is somewhat insufficient in hardness after the quenching-tempering thereof. It is therefore necessary to increase the hardness in order to further suppress the printing wear. It is also found that the material disclosed in Japanese Patent Application No. 63-332156 suffers a difficulty in working when a hot rolled wire material is wire-drawn to a wire diameter of 0.2 to 0.3 mm which is needed for the dot wire. Also, in the latter case, it has been found that the bending and the deterioration in toughness are apt to occur due to a large amount of residual austenite upon quenching.